Apparatus for subjecting a load to a constant temperature below 100°
This is a continuation-in-part of application Ser. No. 901,230, filed Aug. 28, 1986 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an apparatus of the type used to subject culture media and/or instruments to temperatures below 100° C. for predetermined time periods, also referred to as low temperature sterilization. 
     An important aspect of providing low temperature sterilization is the maintaining of a substantially constant and uniform temperature within the chamber, preferably within plus or minus 0.5° C. 
     One method in the prior art for providing low temperature sterilization is described in U.S. Pat. No. 4,395,383. Such prior art devices waste large amounts of energy in the form of condensed steam which is continuously passed to drain. Additionally, this device requires numerous control and sensing mechanisms in order to get the chamber to its working temperature and maintaining at such temperature. 
     Applicants invention provides a simple, efficient and economical method and apparatus for providing a low temperature sterilizing apparatus. 
     SUMMARY OF THE INVENTION 
     Method and apparatus for providing a low temperature environment at a substantially constant and uniform target temperature. For the purpose of this invention, low temperature sterilization shall mean temperature below 100° C., preferably in the range of about 76° C to 99° C. A closed chamber is provided with an inlet, an outlet and a temperature sensor Steam is introduced into the chamber through the inlet for providing heat to bring the enviroment to the target temperature. The chamber is operated at subatmospheric pressure by intermittently applying a vacuum to the outlet to regulate the steam about its saturation point for the preselected target temperature. 
     The apparatus further includes control means which control the intermittent aplication of vacuum to the chamber. The vacuum is applied only as required to maintain the steam in a saturated condition. In one embodiment, the admission of steam and the application of vacuum are both controlled solely according to changes in the chamber temperature. In a second embodiment, the admission of steam is controlled according to chamber temperature, but the application of vacuum is controlled according to whether the steam in the chamber is in a saturated condition. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the preferred apparatus embodying the present invention; 
     FIG. 2 is a schematic diagram of a modified apparatus embodying the present invention; 
     FIG. 3 is a schematic diagram of yet another embodiment of the apparatus made in accordance with the present invention; 
     FIG. 4 is a schematic diagram of yet still another apparatus made in accordance with the present invention; 
     FIG. 5 is a schematic diagram of a further embodiment of an apparatus according to the present invention; and 
     FIG. 6 is a schematic diagram of another embodiment of the apparatus made in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates in schematic form an apparatus for use according to the present invention. A sterilizing chamber 10 is appropriately provided with a door (not shown) which may be opened to allow placement of articles therein. The door should be capable of providing an appropriate seal when closed. A jacket chamber 12 is provided which completely surrounds sterilization chamber 10. The jacket chamber 12 has an inlet 14 connected to a steam source which provides steam at a temperature greater than 100° C. and supra-atmospheric pressure. The jacket chamber 12 also has an outlet 16 which is connected by appropriate piping to the inlet 18 of chamber 10. Sterilization chamber 10 is provided with an outlet 20 which is connected to a water ejector 22 through check valve 21. The flow of water through ejector 22 is controlled by solenoid valve 25. When ejector 22 is operated a vacuum will be applied to outlet 20 which of course will provide a vacuum to chamber 10. A temperature sensor 24 is disposed in the jacket chamber 12 for monitoring the temperature of steam therein. The temperature of the steam in jacket chamber 12 is directly related to the temperature of the steam in chamber 10. Jacket chamber 12 has a second outlet 23 which is connected to ejector 22 through needle valve 27. The ejector 22 also applies a vacuum to jacket chamber 12 in the same manner as chamber 10. The condensed steam from chambers 10 and 12 along with the water used to operate ejector 22 goes to drain. Needle valve 27 is provided to restrict the flow through the jacket outlet 23 thereby allowing the great majority of the steam to flow through chamber 10. 
     In a preferred embodiment, a control means such as a microprocessor is constructed to control the water ejector 22 such that the ejector will not operate when steam is being admitted to the jacket chamber 12. Temperature sensor 24 is connected to the microprocessor which opens solenoid valve 26 so as to allow steam to enter into jacket chamber 12 when the temperature sensed to chamber 12 is below a lower boundary value. The microprocessor turns on the vacuum source when the temperature in the chamber rises above an upper boundary temperature for chamber 10. The vacuum source remains on until the steam in the chamber 10 reaches its saturation pressure point for the target temperature, after which as the ejector continues to operate the further reduction in chamber pressure leads to expansion and cooling of the steam therein. When the temperature sensed within the chamber falls below the lower boundary value, the ejector is deactivated and the steam value is again opened to admit steam to the chamber. 
     Therefore, when steam is flowing into the jacket chamber 12, water flow to the ejector is shut off and when steam is shut off, water will flow through the ejector 22 thereby pulling a vacuum on sterilizer chamber 10 and jacket chamber 12. The control means so constructed brings steam within the chamber to its saturation pressure for a preselected target temperature which falls between the upper and lower boundary values, and maintains the steam in that condition, without any requirement for monitoring the chamber pressure. 
     When the apparatus is started from a cold start, steam is injected into the jacket chamber 12. The steam condenses and the air and water are discharged through needle valve 27. When the temperature sensor senses that the steam in jacket chamber 12 has reached the upper boundary value, the microprocessor will shut off the steam supply by closing solenoid valve 26 and the ejector 22 will be activated by opening valve 25 thereby drawing off condensate and lowering the temperature and pressure of the steam within the sterilization chamber 10 and jacket chamber 12. The lower pressure causes the steam in the chambers 10 and 12 to expand, thus lowering the temperature. The temperature sensor 24 senses this lower temperature. When the temperature in the jacket chamber 12, drops below the preselected temperature, the microprocessor deactivator the ejector and reopens solenoid 26 allowing additional steam to enter. This process is repeated until the sterilization chamber 16 and jacket chamber 12 are warmed up to the preselected working temperature. During the initial warm up stage, the ejector will be running substantially longer than the steam flowing into the chambers 10 and 12. Once the operating temperature in chamber 10 is achieved the alternate steam and ejector cycles continue, but the amount of time the ejector is operated and the time period the steam is allowed to enter chambers 10 and 12 is preferably kept substantially the same. Applicants have found that in order to obtain this substantially equal balance, the solenoid 26 has a 3/32 inch (2.38 mm) orifice and the ejector 22 is designed to evacute 100 cubic foot tank to 15 inches of Mercury absolute (Hg. Abs.) in 74 minutes. The process of allowing steam into chamber 10 and applying a vacuum to chamber 10 will continue for as long as has been programmed in the microprocessor for the desired preselected temperature. 
     The present invention requires only to monitor a single temperature which can be easily done by a single temperature sensor. This allows ease in selecting different operating temperatures to be controlled. Additionally, the controlling of steam and vacuum is a function of a single parameter. Regulating the steam about its saturation point effect uniformity of temperatures throughout the chamber 10, minimizing the danger of overheating the contents. Alternating the entrance of steam into chamber 10 and applying a vacuum to chamber 10 minimizes the amount of energy (heat loss) that goes to drain. 
     In the preferred embodiment a small amount of air is introduced into chamber 10. In the particular embodiment illustrated air is introduced into the steam line going to inlet 14. Air enters filter 30 through orifice 32 to steam line 34. Orifice 32 allows only a very small amount of air to enter. Applicants have found that an orifice having a diameter of about 5.08 mm (0.02 inches) is adequate. 
     The apparatus of FIG. 1 has been found to maintain the temperature in a chamber 10 size 20&#34;×20&#34;×28&#34; to within ±0.5° C. of the target temperature. Steam was imtroduced into jacket chamber 12 at a temperature of 150° C. and pressure of 60 psi. An operating temperature of 80° C. was selected. The temperature sensor 24 in jacket chamber 12 sends the appropriate information to the microprocessor. The microprocessor was programmed so that when a temperature below of 80° C. was sensed in chamber 10, valve 26 was opened allowing steam to enter jacket chamber 12. In the particular apparatus used there was about a two degree difference between the temperature in chamber 10 and the temperature sensed by temperature sensor 24 in jacket chamber 12. Therefore, the temperature sensor 24 measured 82° C. when the temperature in chamber 10 was 80° C. The microprocessor was programmed accordingly. When the temperature sensor 24 measured a temperature of 83° C., the valve 26 was closed and valve 25 was opened thereby applying a vacuum to chamber 10. The apparatus then cycled between allowing steam to enter and applying a vacuum. Once the chamber 10 reached the preselected operating temperature, the cycle time for allowing steam to enter and applying a vacuum was more equal. In the embodiment illustrated the steam cycle was about two (2) seconds and the vacuum cycle was about four (4) seconds. 
     FIGS. 2, 3, 4, 5 and 6 illustrate various modified forms of the present invention, identical numerals indicating like parts. Referring to FIG. 2 there is illustrated an embodiment similar to FIG. 1 except that instead of steam entering into chamber 10 from jacket chamber 12, the steam enters directly into inlet 18 of chamber 10 and directly into jacket chamber 12 through a second solenoid 126. Additionally, a second temperature sensor 124 is provided in chamber 10 and is appropriately connected to the microprocessor. The jacket chamber 12 has only a single outlet 28 connected to a separate ejector 122 and the sterilization chamber 10 is connected to its own ejector 22 through outlet 20. This particular embodiment operates substantially identical as the apparatus of FIG. 1 except the jacket chamber 12 and sterilization chamber 10 operates independently of each other so that each may operate its own preselected temperature. 
     FIG. 3 is an embodiment similar to that illustrated in FIG. 1 except the jacket chamber 12 is omitted and the temperature sensor 24 is placed in the sterilization chamber 10. In this embodiment, it is recommended that the outer surface of sterilization chamber be adequately insulated. This embodiment also operates in the same manner as the apparatus of FIG. 1. 
     Referring to FIG. 4 there is illustrated an alternate preferred embodiments of the present invention similar to that of FIG. 1. As in the embodiment of FIG. 1, the microprocessor admits steam into the jacket chamber 12 and sterilization chamber 10 when the temperature falls below the preset lower boundary value. However, the control means governing the application of vacuum to chambers 10 and 12 is steam traps 50 connected to the outlets 20 and 23, instead of a microprocessor. The steam traps 50 of the embodiment illustrated are thermostatic pressure balanced thermostatic traps which closely follow the steam saturation curve. From a cold start, steam is injected into the jacket chamber and condenses. As the chamber 10 warms up, the traps 50 remain open due to the condensate going through them. The open traps 50 allow the ejector 22 to lower the pressure in chambers 10 and 12 which lowers the saturation temperature of the steam. When the temperature in the chamber reaches the preset upper boundary value, the microprocessor turns off the steam. The traps stay open, allowing evacuation of the chamber for as long as condensation continues to occur, i.e. until the pressure in chambers 10 and 12 is reduced to the saturation point for the preset temperature. When the pressure reaches that point, condensation ceases and the traps close, holding the pressure substantially constant. 
     Eventually the temperature drops below the lower boundary value, triggering the admission of steam to the chambers. The resultant increase in pressure and temperature leads to condensation of the steam, which in turn causes the traps to open, repeating the cycle. In this particular embodiment temperature sensor 24 measures the temperature of the drain as is normally done in sterilization. 
     The embodiment of FIG. 5 operates in the same manner as the apparatus in FIG. 4 however as in FIG. 2 chamber 10 is independently connected to the steam source. The steam traps as in FIG. 4 are used to determine when the vacuum is to be applied to the sterilization chamber 10 and jacket chamber 12. Additionally in this embodiment, as in FIG. 2, the chamber and jacket chambers may be kept at different preselected temperatures. 
     The embodiment illustrated in FIG. 6 also uses a steam trap for controlling the vacuum applied to the chamber 10, however, the jacket chamber 12 is omitted as in FIG. 3. 
     While the present invention has been described with regard to the particular embodiments illustrated, various modifications may be made without departing from the scope of the present invention.